Various means have been recently taken up for reducing the rolling resistance of a tire (for improving rolling resistance property) or for improving the steering stability of a vehicle in order to save energy. As the means, it is mentioned that a tire tread is made as double layer structure (inner layer and surface layer) and a rubber composition exhibiting superior rolling resistance and steering stability is used for a base tread being its inner layer. Further, it is mentioned that a rubber composition superior in rolling resistance and durability (crack growth resistance) is used in the sidewall, clinch and insulation and a rubber composition superior in abrasion resistance is used in a clinch.
In order to improve the rolling resistance and steering stability, there are disclosed technology that reduces loss tangent (tan δ) and reduce the rolling resistance of a tire by including a modified butadiene rubber and carbon black as a rubber composition for a base tread and technology that improves complex elastic modulus (E*) by including a polybutadiene rubber including 1,2-syndiotactic polybutadiene crystals and further reduces tan δ to improve the rolling resistance and steering stability by reducing the content of carbon black (for example, refer to Japanese Unexamined Patent Publication No. 2006-124503).
However, when a rubber having butadiene skeleton and a rubber having isoprene skeleton are included at same level in like manner as the combination of a butadiene rubber and a natural rubber as a diene rubber component, cure reversion occurs easily and it causes the reduction of E* and the increase of tan δ.
Consequently, although it is necessary for suppressing cure reversion that a styrene-butadiene rubber is included in a diene rubber component, the reduction of tan δ is not adequate only by including reinforcing fillers such as low reinforcing carbon black and silica in the styrene-butadiene rubber or by lessening the content of the reinforcing fillers.
Further, when a rubber composition for a base tread disclosed in Japanese Unexamined Patent Publication No. 2006-124503 is applied for a sidewall, a clinch and an insulation, the compatibility of low heat build-up property and elongation at break (durability) has been susceptible to improvement.